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Dictionary

Global Grid-Storage Policies

Efficiency is a technical term for the conversion of one energy from one form to another. Energy is never lost (Second Law of Thermodynamics), so if a fuel is burnt entirely, 100% has been converted to other energy forms.

The power generation efficiency is the rating of an electricity generating power station, as a percentage of the ratio of effective electrical output over energy input.

An important principle of thermodynamics states that no conversion is perfect – there will always be some loss to another form of energy. The amount of energy available to the animal’s body is much less than the total amount of energy from the sun to make the glucose in the plant. A ball does not bounce back to the same height as before, because some of the energy is ‘lost’ to heat and sound. The total amount of energy is always the same, but not all of the energy can be converted to a single form. The percentage that can be converted is known as efficiency.

η = output/input

This loss of 'usable' energy is called degradation of energy.

Efficiency is a technical term for the conversion of one energy from one form to another. Energy is never lost (Second Law of Thermodynamics), so if a fuel is burnt entirely, 100% has been converted to other energy forms. However, much of the new energy may be 'unusable', such as sound. If heat is the objective of the conversion, as it is in a boiler and turbine power generator, the efficiency would be a measure of how much electrical power results from the burning of an amount of fuel.

Burning oil in a power plant has the purpose of heating water, which is converted to steam, which drives a turbine, which generates electricity. Burning petrol in a car considers efficiency of conversion to be the kinetic energy of motion, and heat is 'unusable', so is lost to the purpose of the conversion.

Energy sourceEnergy typeCurrent rangeTheoretical max.
Windkinetic30-50%59%
Photovoltaicradiative15-20%90%
Hydropowergravitational80-90%90%
Fuel cellchemical70-80%85%
Gas turbinechemical30-40%40%
Combined cycle*chemical and thermal40-60%60%
World TotalAll types39% gross33% net

* Two stage production: gas turbine then steam turbine

Heating Systems in New Residences

New accommodation in Germany (%)

YearMethaneHeat pumpDistrict heatingElectricityHeating oilWood/Pellets
200076.70.87.0<113.40
200574.05.48.6<16.43.0
201050.223.514.6<11.85.0
201549.420.420.0<10.65.5

Data source: AG Energiebilanzen e.V. 3-Quarter Report 2015

Laws of Thermodynamics

"Thermo-dynamics is the subject of the relation of heat to forces acting between contiguous parts of bodies, and the relation of heat to electrical agency" (Lord Kelvin). The four laws of thermodynamics were derived from the work of, among others, Sadi Carnot, Émile Clapeyron, Julius Mayer, Hermann Hess, und Rudolf Clausius. They explain heat exchange, entropy, and how substances and machines behave under energy transformation.

Carnot heat engine
The Carnot heat engine explains why all energy conversions are inefficient

The four Laws of Thermodynamics:

0. (Zeroth) If two systems are each in thermal equilibrium with a third, they are also in thermal equilibrium with each other.

1. The internal energy of an isolated system is constant. The law of conservation of energy is a consequence, and states that energy can be transformed from one form to another, but cannot be created or destroyed.

2. Heat cannot spontaneously flow from a colder location to a hotter location. Rudolf Clausius: There is no change in state, the only result of which is the transmission of heat from a body of a lower temperature to a higher temperature body. Entropy is a measure of the loss of order (distribution of energy) and tends to increase with time. This leads to the irreversibility of reactions.

3. As a system approaches absolute zero (0 K or −273.15 °C), all processes cease and the entropy of the system approaches a minimum value. Entropy can not be destroyed. However, entropy may arise in the system. Energy degradation results from irreversible processes.

Intensive quantities: temperature T, pressure ρ, concentration n, chemical potential μ

Extensive quantities: internal energy U, entropy S, volume V and particle number N